Spherical symmetry in the kilonova AT2017gfo/GW170817
Albert Sneppen1,2, Darach Watson3,4, Andreas Bauswein5
1Cosmic Dawn Center (DAWN), Copenhagen, Denmark. a.sneppen@gmail.com.
Nature
|February 15, 2023
Summary
Neutron star mergers create kilonovae. Analysis of AT2017gfo reveals a highly spherical kilonova, challenging models that predict aspherical ejecta and suggesting new physics may be involved.
Area of Science:
- Astrophysics
- Nuclear Physics
- Gravitational Wave Astronomy
Background:
- Neutron star mergers produce kilonovae, observable as heavy-element enriched fireballs.
- Kilonova geometry offers insights into ultra-dense matter and black hole formation energetics.
- Existing hydrodynamical models predominantly predict aspherical ejecta from mergers.
Purpose of the Study:
- To determine the geometry of the kilonova AT2017gfo.
- To investigate the factors influencing kilonova ejecta shape.
- To reconcile observational data with theoretical merger models.
Main Methods:
- Analysis of the strontium (Sr+) P Cygni spectral feature in AT2017gfo.
- Characterization of the kilonova's blackbody spectrum.
- Line shape analysis combined with the known inclination angle of the source.
Main Results:
- The kilonova AT2017gfo exhibited a highly spherical geometry in its early stages.
- Independent analyses using spectral features and inclination angle confirmed this sphericity.
- Radioactive decay alone cannot account for the observed spherical ejecta.
Conclusions:
- The spherical geometry of AT2017gfo suggests energy injection from a magnetar wind or black hole-disk jet.
- An additional, yet unidentified, process is likely required for uniform element distribution in the ejecta.
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